This invention relates to electrodes which are useful in electrochemical generators and, more particularly, to zinc electrodes for use in rechargeable electrochemical cells.
It is believed that shape change occurs in the negative plates or electrodes of cells which include zinc electrodes in alkaline electrolytes, e.g., silver/zinc and nickel/zinc cells, whenever any part of the negative plate becomes zinc limiting. The latter appears to occur at any discontinuity in the negative plate such as the plate edges and fissures within the plate. At the locations of discontinuity in the negative plate, the zincate concentration becomes dilute during charge while at other locations of the negative plate, e.g., the center, there can be an excess of zinc oxide which will maintain the electrolyte at the latter location at saturation with respect to zincate. Thus, a concentration cell can develop between the locations in the negative plate which are dilute in zincate and those locations which are associated with a high concentration of zincate. Such concentration cells can result in a transfer of zincate from one portion of the negative plate to another thereby producing shape change.
A number of methods to reduce or eliminate electrode shape change in the aforementioned cells presently exist. One of these methods involves cell construction features which function to reduce the possibility of concentration polarization build-up at the negative electrode during charge. Such a method is described in U.S. Pat. No. 3,505,115, issued Apr. 7, 1970, and assigned to the instant assignee. This patent describes the sizing of the negative plate so that it is larger than and overlaps the positive electrode.
Another method involves preventing the solubilization of the zinc as it is anodized. Such a method is described in U.S. Pat. No. 3,536,537, issued Feb. 3, 1970, and assigned to the instant assignee. The latter patent teaches the addition of a small quantity of a fluorocarbon polymer to the negative zinc electrode.
Although the methods described in the aforementioned patents provide a substantial increase in cell life, they do not completely eliminate the problems or zinc electrode edge erosion and shape change, particularly after prolonged cycling of the electrode. Additionally, whereas cells built with extended edge negative electrodes and fluorocarbon impregnated negative electrodes exhibit the benefits (although to a lesser degree) of the herein-described invention only after numerous charge/discharge cycles, e.g., on the order of 90 such cycles, cells containing negative electrodes as described herein exhibit improved capacity maintenance after only a few charge/ discharge cycles, e.g., on the order of (3) cycles. Therefore, there remains a need for a technique which will provide further improvements in zinc electrode edge erosion and shape change.
The incorporation of various types of fibers, both organic and inorganic, in both positive and negative electrodes for the purpose of providing a physically stronger electrode is described in U.S. Pat. No. 3,271,195, issued Sept. 6, 1966, which is assigned to the instant assignee. Although the herein-described invention can provide improvements in the strength of electrodes similar to that described in the latter patent, the fibrous material identified in the latter patent are not capable of providing the improvements in cell capacity maintenance and electrode shape change which are obtainable from the herein-described invention.
U.S. Pat. No. 3,476,601, issued Nov. 4, 1969 discloses the use of about 2% to about 50% by weight of a titanate compound in or against either electrode in a high density battery for the purpose of mechanically strengthening the electrodes. There is no recognition in the patent of the electrochemical improvements obtainable from such titanate compounds. This lack of recognition is reflected in both the concentration range given for the titanate compound and in the location of the latter.
The charging current densities normally employed for charging silver/zinc cells range between 1.5 and 3.0 ma/cm.sup.2. However, the limiting current density (LCD) for a cell incorporating 5% (wt.) titanate in the zinc electrode is only 0.77 ma/cm.sup.2. Since the LCD decreases with increasing percent titanate in the negative electrode, it will be understood that the titanate range recommended in U.S. Pat. No. 3,476,601 is unrealistic in electrochemical terms although it may be quite acceptable in mechanical terms. In fact, mechanically strengthening the negative (and positive) electrode seems to have been the only object of the U.S. Pat. No. 3,476,601 since it discloses either incorporation of the titanate in the electrode or placement of the titanate against the electrode in order to achieve greater mechanical strength. As is well known, placement of the titanate against an electrode is not normally recommended for improvement in electrochemical properties.